For Ohio’s bird enthusiasts, May is prime time, as the arrival of northbound migrants adds color and song to the landscape. No group of birds exemplifies migration season more than the wood warblers. That family of birds, the Parulidae, includes about ten dozen species. Nearly a third of those appear in Ohio in May, and individuals from about a dozen parulid species stay in Greene and Montgomery Counties to breed.
The parulids arriving in Ohio share a number of traits. All are small, about the size of a chickadee. They have pointy little beaks that facilitate their insect diet. And they are migratory, breeding in the US and/or Canada and wintering at latitudes ranging from the southern US to the tropics of Central and South America. Despite those commonalities, the species are readily identifiable by sight and sound. Biologists refer to a phenomenon like this—an evolutionary lineage that has diversified into a bunch of related species—as an adaptive radiation. And adaptive radiations raise two big questions: Why did they arise? And how are they maintained?
The origins of an adaptive radiation can be illustrated using a famous example: the finches studied by Charles Darwin. In that case, finches from mainland South America occasionally blew out to sea and landed on one of the numerous Galapagos Islands far off shore. Each of those islands provided unique biological and physical conditions. And so, over time, the combination of mutations and responses to environmental factors (weather, food availability, etc.) led to an accumulation of changes (in genetics, behavior, and form and function) over successive generations. This process happened repeatedly, and the result was an adaptive radiation—numerous, related finch species inhabiting the island group, differing from each other and from the original mainland species in a variety of characteristics.
The circumstances that produced isolated populations of ancestral parulid warblers were different—not islands separated by stretches of ocean, but temperate, insect-rich habitat isolated by incursions of colder, inhospitable conditions. Scientists are still collecting evidence—from DNA sequencing to comparisons of flight calls—to test ideas on the exact timing and conditions that induced the adaptive radiation. Was it during the Pleistocene Ice Ages or just before? Were the ancestral warblers tropical and non-migratory or temperate and migratory? But under any of those scenarios, the critical circumstances were a changing and fragmented landscape induced by fluctuations in climate that repeatedly separated and isolated warbler populations in North America. We now see the result: lots of warbler species!
As an adaptive radiation evolves, the question then is whether it can persist. How do the related species coexist in a common environment, and what prevents them from interbreeding and “washing out” any accumulated differences? Ecologists believe that for species to stably coexist within a shared community, they need to somehow use the environment differently from each other-, a phenomenon called resource partitioning. Indeed, the term “adaptive” radiation implies that the various species have adapted new traits that might separate them from each other. Galapagos Island finch species, for example, have diverged in size and, notably, in beak morphology such that diets of particular species now range from small seeds to large, and from seeds to fruits to insects.
As noted above, parulid warblers all are small, insect-eating birds; there is little diversification of body structure or diet along the lines of Darwin’s finches. So how do they partition the environment? In his pioneering research in the 1950s, Robert MacArthur, one of the founders of the modern science of ecology, observed that a group of five warbler species inhabiting Maine evergreens separated from each other using behavior: what parts of the tree they prefer (e.g., high up vs. low down, interior vs. exterior), foraging habits (e.g., whether or not they fly out after insects), and timing of peak activity. MacArthur generalized those observations into theories of species coexistence and resource partitioning.
Parulid warblers maintain species diversity in another obvious way, too. That is, members of a species recognize each other using unique combinations of visual and auditory signals. Some of the species names reflect those visual patterns: black-throated green warbler; blue-winged warbler; black and white warbler. And the songs are equally distinct. As described by David Sibley: “zeeeeeeeeeeee-tsup” for the northern parula, Setophaga americana, which breeds locally; “tsi tsi tsi tsi tsi ti ti ti ti ti seeeeee” for the beautiful Blackburnian warbler, Setophaga fusca, which migrates through Ohio.
Three locally breeding species, all small, colorful, insectivores in the genus Setophaga, illustrate these patterns of differentiation. The yellow warbler, Setophaga petechia, is bright yellow, with red streaking on the breast in males. Yellow warblers prefer shrubby wetlands, and the males can be heard singing, “sweet sweet ti ti ti to soo,” from the branches of water-side willows. Prairie warblers, Setophaga discolor, also are mostly yellow, though not so bright as yellow warblers. They also like shrubby habitat but prefer second-growth meadows and woods rather than wetlands. “Zooo zoo zo zozozozoZEEET!” Yellow-throated warblers, Setophaga dominica, have bodies handsomely striped in black and white, with brilliant yellow throats. They are closely associated with sycamore trees, and watching them sing from the high treetops, “teedl teedl teedl teedl teedl teedl teedl tew tew twee,” is a great way to earn a case of “warbler neck.”
Warblers are small enough that it’s easy to overlook them. But the arrival of these colorful, musical, active birds is highly anticipated each year, and the combination of colors, songs, and habits provides a key to finding them. May, especially early in the month before trees fully leaf out, is prime time for warblering. To quote the magnolia warbler, Setophaga magnolia: “sweeter sweeter SWEETEST!!”
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.